Principles of Inheritance and Variation: Class 12 Biology Practice Questions

A genetics student, Priya, is studying flower colour inheritance in a plant species. She crosses a true-breeding red-flowered plant with a true-breeding white-flowered plant. All F1 plants show pink flowers. When Priya self-pollinates the F1 plants, she obtains 240 plants in the F2 generation — 60 red-flowered, 120 pink-flowered, and 60 white-flowered.

In a separate experiment, Priya's classmate Rohan studies ABO blood group inheritance in humans. He notes that his father has blood group A (genotype I^A i) and his mother has blood group B (genotype I^B i). Rohan wants to predict all possible blood groups in their children.

Read the following passage carefully and answer the questions that follow:

A genetics student, Priya, is studying flower colour inheritance in a plant species. She crosses a true-breeding red-flowered plant with a true-breeding white-flowered plant. All F1 plants show pink flowers. When Priya self-pollinates the F1 plants, she obtains 240 plants in the F2 generation — 60 red-flowered, 120 pink-flowered, and 60 white-flowered.

In a separate experiment, Priya's classmate Rohan studies ABO blood group inheritance in humans. He notes that his father has blood group A (genotype I^A i) and his mother has blood group B (genotype I^B i). Rohan wants to predict all possible blood groups in their children.

(a) Identify the type of inheritance observed in Priya's experiment. Give ONE reason to justify your answer. (1 mark)
(b) Write the genotypes of the true-breeding red-flowered and white-flowered parent plants. Using a Punnett square, show the F1 and F2 generations, and state the genotypic and phenotypic ratios obtained in F2. (2 marks)
(c) From Rohan's family cross (I^A i × I^B i), list ALL possible blood groups (phenotypes) that can appear in their children. Which blood group demonstrates co-dominance, and why? (1 mark)

A genetics teacher presented the following family pedigree to her Class XII students and asked them to analyse it:

Generation I: Carrier female (I-1) × Unaffected male (I-2)
Generation II: Affected male (II-1), Unaffected female (II-2), Unaffected male (II-3)
Generation III: Affected male (III-1) [son of II-2 and her unaffected husband]

The trait shown in the pedigree skips generations, affects more males than females, and unaffected females can pass it to their sons. The teacher asked the students to answer the following questions based on the pedigree.

A genetics teacher presented the following family pedigree to her Class XII students and asked them to analyse it:

Generation I: Carrier female (I-1) × Unaffected male (I-2)
Generation II: Affected male (II-1), Unaffected female (II-2), Unaffected male (II-3)
Generation III: Affected male (III-1) [son of II-2 and her unaffected husband]

The trait shown in the pedigree skips generations, affects more males than females, and unaffected females can pass it to their sons.

(a) Identify the pattern of inheritance shown in this pedigree. Give TWO reasons to justify your answer. (2 marks)
(b) Write the genotypes of Individual I-1 (carrier female) and Individual II-1 (affected male) using proper genetic notation. (1 mark)
(c) Individual II-2 (unaffected female from Generation II) marries an unaffected male. Draw a Punnett square to show the possible genotypes of their children and state the probability that their son will be affected. (1 mark)

A genetics counsellor is examining the family history of the Sharma family. Mrs. Sharma has normal vision, but her father was colour blind. Mr. Sharma has normal vision and no history of colour blindness in his family. They have three children: a colour-blind son (Arjun), a daughter with normal vision (Priya), and another son with normal vision (Rohit).

Colour blindness is an X-linked recessive trait. The allele for normal vision is represented as X^N and the allele for colour blindness is represented as X^n.

Read the following case and answer the questions that follow:

A genetics counsellor is examining the family history of the Sharma family. Mrs. Sharma has normal vision, but her father was colour blind. Mr. Sharma has normal vision and no history of colour blindness in his family. They have three children: a colour-blind son (Arjun), a daughter with normal vision (Priya), and another son with normal vision (Rohit).

Colour blindness is an X-linked recessive trait. The allele for normal vision is represented as X^N and the allele for colour blindness is represented as X^n.

(a) What is the genotype of Mrs. Sharma? Give a reason for your answer. [1]
(b) Show with the help of a Punnett square the cross between Mr. and Mrs. Sharma. [2]
(c) What is the probability that Priya is a carrier of colour blindness? Justify your answer. [1]

A genetics teacher showed her students a pedigree chart of a family across three generations. In Generation I, a normal male married a normal female. In Generation II, they had two sons and two daughters — one son was colour blind, the other son was normal, one daughter was normal (who later married a normal male), and the other daughter was normal. In Generation III, the normal daughter from Generation II had two children — a colour blind son and a normal daughter.

The teacher then asked the students to analyse the pattern of inheritance and draw the relevant genetic cross.

Read the following case and answer the questions that follow:

A genetics teacher showed her students a pedigree chart of a family across three generations. In Generation I, a normal male married a normal female. In Generation II, they had two sons and two daughters — one son was colour blind, the other son was normal, one daughter was normal (who later married a normal male), and the other daughter was normal. In Generation III, the normal daughter from Generation II had two children — a colour blind son and a normal daughter.

The teacher then asked the students to analyse the pattern of inheritance and draw the relevant genetic cross.

(i) Identify the pattern of inheritance shown in the pedigree chart. Give one reason to support your answer. (1 mark)

(ii) What would be the genotype of the Generation I female (mother)? Justify your answer. (1 mark)

(iii) Draw a labelled Punnett square showing the cross between the Generation II normal daughter and her normal husband. Show the genotypes of all offspring. (2 marks)

Which of the following statements about linkage and recombination is INCORRECT?

Assertion (A): In a cross between a carrier female (X^H X^h) and a normal male (X^H Y), 50% of the sons are expected to be haemophilic.
Reason (R): Haemophilia is an X-linked recessive disorder in which the defective allele is transmitted from carrier mother to sons through criss-cross inheritance.

A farmer notices that when he crosses two true-breeding varieties of snapdragon (Antirrhinum majus) — one with red flowers and one with white flowers — all F1 plants have pink flowers. He then self-pollinates the F1 plants to obtain F2. A pest destroys all white-flowered F2 plants before he can count them. Which of the following correctly identifies the genotypic ratio of the SURVIVING F2 plants?

Assertion (A): In humans, the sex of the offspring is determined by the type of sperm that fertilises the egg.
Reason (R): Females are homogametic (XX) and produce only one type of egg, while males are heterogametic (XY) and produce two types of sperm.

A researcher studying a rare autosomal recessive disorder in a family notices the following: the affected child (III-1) has two phenotypically normal parents (II-1 and II-2). The maternal grandfather (I-1) is affected, but the maternal grandmother (I-2) and paternal grandparents (I-3 and I-4) are all phenotypically normal. The researcher correctly predicts the probability that the next child born to II-1 and II-2 will be an affected daughter.

Which of the following gives the correct genotypes of II-1 and II-2, AND the probability of an affected daughter in their next child?

A farmer crosses two true-breeding pea plants — one with round seeds (RR) and one with wrinkled seeds (rr). He plants all the F1 seeds and allows them to self-pollinate to get F2 seeds. He then randomly picks ONE seed from the F2 generation. What is the probability that this seed is wrinkled?

Study the incomplete Punnett square given below showing a monohybrid cross between two pea plants heterozygous for seed colour (Yellow, Y is dominant over green, y).

Y y
┌─────────┬─────────┐
Y │ YY │ │
├─────────┼─────────┤
y │ │ yy │
└─────────┴─────────┘

(a) Complete the Punnett square by filling in the two missing genotypes.
(b) What will be the phenotypic ratio of the offspring from this cross?

A genetics student performed two separate monohybrid crosses using pea plants:

Cross P: Tall (TT) × Dwarf (tt) → All F1 plants were Tall
Cross Q: When two F1 plants from Cross P were crossed with each other, the F2 generation showed both Tall and Dwarf plants.

(a) What principle of Mendel does the result of Cross P demonstrate? (1 mark)
(b) In Cross Q, a student claims that the ratio of Tall : Dwarf plants in F2 will be 3:1. Is the student correct? Justify your answer in one sentence. (1 mark)

A woman with normal vision but whose father was colour blind marries a man with normal vision. What is the probability that their son will be colour blind? Give the genotypes of both parents and justify your answer.

A genetics student is analysing the pedigree of a family in which a male child is born with Klinefelter syndrome. During meiosis in the mother, a non-disjunction event occurred.
(i) Write the genotype (in chromosomal notation) of this affected child.
(ii) Identify the specific stage of maternal meiosis at which non-disjunction must have occurred to produce this child, and justify your answer in one sentence.

In humans, a person is found to have 47 chromosomes with the karyotype 45 + XXY. (i) Name the chromosomal disorder this person is suffering from. (ii) State one characteristic feature of this disorder.

A genetics student is analysing a pedigree of a family in which a particular disorder appears. She observes the following: (i) The disorder appears in every generation. (ii) Both males and females are affected with equal frequency. (iii) An affected father has an affected son. Based on these observations, she concludes that the disorder follows autosomal dominant inheritance. However, her friend argues that the disorder could also be X-linked dominant. The student then finds one additional piece of evidence that conclusively rules out X-linked dominant inheritance. (a) Identify the ONE piece of evidence (from the three observations given) that conclusively rules out X-linked dominant inheritance. Give a reason. (b) Work out the cross between an affected heterozygous father (autosomal dominant) and a normal mother to show the expected genotypic and phenotypic ratios in the offspring.

Priya is a healthy woman whose brother has sickle cell anaemia. She wants to know whether she is a carrier of the sickle cell trait before planning her family. Her husband Arjun has no family history of the disease.

(a) State the inheritance pattern of sickle cell anaemia and identify Priya's possible genotypes. (1 mark)
(b) If Priya is confirmed to be a carrier and she marries Arjun (who is normal, non-carrier), draw a cross to show the expected genotypes and phenotypes of their offspring. State the genotypic ratio. (2 marks)

A genetics student performed a dihybrid cross between two true-breeding plants — one with round, yellow seeds (RRYY) and another with wrinkled, green seeds (rryy). The F₁ plants were then test-crossed.

(a) Write the genotype and phenotype of the F₁ generation. (1 mark)
(b) Give the genotypic and phenotypic ratios expected in the test-cross progeny, assuming both genes are on different (non-homologous) chromosomes. (1 mark)
(c) If, instead, the two genes were found to be located on the same chromosome and showed incomplete linkage, how would the test-cross results differ from those obtained in (b)? Give one reason for your answer. (1 mark)

In Antirrhinum (snapdragon), flower colour shows incomplete dominance. When a pure-breeding red-flowered plant is crossed with a pure-breeding white-flowered plant, the F₁ plants all bear pink flowers. (i) What are the genotype and phenotype of the F₁ plants? (ii) Write the cross showing the genotypes of all plants when F₁ pink-flowered plants are selfed (F₁ × F₁). Show this cross using a Punnett square. (iii) State the phenotypic ratio of the F₂ generation.

A genetics student is analysing data from two separate crosses in Drosophila melanogaster:

Cross 1: A homozygous wild-type female (grey body, normal wings) was crossed with a male having black body and vestigial wings. The F1 females were then test-crossed with black-bodied, vestigial-winged males. Expected result if genes are unlinked: 1:1:1:1. Observed result: predominantly parental combinations with only a small percentage of recombinant classes.

Cross 2: A human carrier female (X^H X^h) married a normal male (X^H Y). Their son was found to be haemophilic.

(i) Explain why Cross 1 does NOT give a 1:1:1:1 ratio, naming the phenomenon responsible and the scientist associated with it. (1 mark)
(ii) Using the Cross 2 data, draw the correct Punnett square and identify the genotype(s) of daughters who would be carriers. (1½ marks)
(iii) In Cross 1, if the recombination frequency between the two loci is found to be 17%, what does this value indicate about the relative positions of the two genes on the chromosome? (½ mark)

A woman with normal vision, whose father was colour blind, marries a man with normal vision. (i) Give the genotypes of the woman and her father. (ii) Work out a cross to show the possible genotypes and phenotypes of their sons and daughters. (iii) A son from this marriage marries a colour-blind woman. What is the probability that their daughter will be colour blind? Give reason.

A woman with normal vision, whose father was colour blind, marries a man with normal vision. With the help of a cross, determine the genotypes of the parents and work out the probability of their children being colour blind. Also, (i) state the pattern of inheritance of colour blindness in humans, and (ii) explain why sons are more frequently affected than daughters in X-linked recessive disorders.

A pure breeding tall pea plant (Pisum sativum) is crossed with a pure breeding dwarf pea plant. Separately, a pure breeding red-flowered Mirabilis jalapa (four o'clock plant) is crossed with a pure breeding white-flowered Mirabilis jalapa. (a) Work out the F₁ and F₂ generations for BOTH crosses using appropriate Punnett squares. (b) State the genotypic ratio and phenotypic ratio obtained in each F₂ generation. (c) Comment on the difference in the pattern of inheritance observed in the two crosses, naming the phenomenon responsible in each case.

A genetics counsellor is studying the inheritance of a rare autosomal recessive disorder called 'Maple Syrup Urine Disease' (MSUD) in a family. The pedigree chart of the family is given below:

Generation I: Carrier female (I-1) × Normal male (I-2)
Generation II: II-1 (affected male), II-2 (normal female — married into family), II-3 (carrier female), II-4 (normal male — married into family)
Generation III: III-1 (normal female), III-2 (affected male), III-3 (normal male)

[Note: Affected individuals are shown as filled symbols; carriers are half-filled; normal individuals are unfilled. Square = male, Circle = female]

The counsellor informs the family that MSUD is caused by a mutation in a gene encoding a metabolic enzyme, and that the disorder follows Mendelian inheritance.

(a) Identify the type of inheritance shown in this pedigree. Give TWO reasons to justify your answer. (2 marks)
(b) Write the genotypes of individuals I-1, I-2, and II-3. Use 'M' for the dominant allele and 'm' for the recessive allele. (1 mark)
(c) The couple II-3 and II-4 (II-4 is a normal, non-carrier male) plan to have a fourth child. Using a Punnett square, show the possible genotypes of their offspring and state the probability that the fourth child will be an affected male. (2 marks)

A geneticist studied a three-generation family in which some members showed a rare autosomal recessive condition called phenylketonuria (PKU). The pedigree is described below:

Generation I: Individual 1 (unaffected male) × Individual 2 (unaffected female)
Generation II: Individual 3 (unaffected male), Individual 4 (affected female), Individual 5 (unaffected female), Individual 6 (unaffected male)
Individual 5 (Generation II) × Individual 6 (Generation II) are married
Generation III: Individual 7 (affected male), Individual 8 (unaffected female), Individual 9 (unaffected male)

Using the information above, answer the following:
(a) Is PKU autosomal or X-linked? Give one reason from the pedigree to justify your answer. (1 mark)
(b) Give the genotypes of Individual 1, Individual 2, Individual 5, and Individual 6. Use 'P' for the dominant allele and 'p' for the recessive allele. (2 marks)
(c) What is the probability that Individual 8 is a carrier of PKU? Show the cross between Individuals 5 and 6 using a Punnett square to support your answer. (2 marks)